1. Field of the Invention
The present invention relates to a sublimation-type thermal color image transfer recording medium for multi-color image formation by use of an n-times-speed mode method, and more specifically to a sublimation-type thermal color image transfer recording medium which is capable of performing multi-color image formation without any reduction in image density even when the n-times-speed mode method is employed and which is advantageous over conventional sublimation-type thermal color image transfer recording media in view of the cost.
2. Discussion of Background
Recently, the demand for full color printing is increasing year by year, and the development of the full-color-printing preparation method is progressed. Recording methods for full color printing include the electrophotographic method, the ink-jet method, and the thermosensitive image transfer method. Of these methods, the thermosensitive image transfer method is most widely employed because of its advantages such as easy maintenance and noiseless operation, over the other methods.
In the thermosensitive image transfer recording method, an image receiving sheet is used, which is a so-called ink sheet and comprises a support and an ink layer formed thereon. The image receiving sheet is superimposed on a thermal image transfer recording medium in such a fashion that the surface of the image receiving sheet comes into contact with the surface of the ink layer.
To the above superimposed recording medium and image receiving sheet, electrically controlled thermal energy is applied to the recording medium side by use of a laser or a thermal head, so that an ink in the heated portion of the recording medium is imagewise transferred from the thermal image transfer recording medium to the image receiving sheet. Thus an image is formed on the image receiving sheet.
The thermosensitive image transfer recording methods can be roughly classified into two types, a thermal fusing image transfer type and a sublimation image transfer type, depending upon the kinds of inks employed.
In a thermal image transfer recording medium for the thermal fusing image transfer type recording method, an ink comprising a coloring agent dispersed in a thermofusible material is used in the recording medium, while in the sublimation image transfer type recording method, an ink comprising a sublimable dye dispersed in a binder resin is employed in an ink layer of a thermal image transfer recording medium.
When the two methods are compared, the sublimation image transfer type has the advantage that halftone images can be obtained without difficulty since a sublimation dye is transferred to the image receiving sheet in the form of individually separated molecules, corresponding to the amount of thermal energy applied from a thermal head. In addition to the above, the sublimation image transfer type recording method has many other advantages over the thermal image transfer type, so that the sublimation image transfer type recording method is considered to be one of the most suitable methods for full-color printing.
The sublimation image transfer recording method, however, has the shortcoming that its running cost is higher than those of the electrophotographic method, the ink jet method and other methods, because (a) it is necessary to employ not only coloring materials which directly contribute to the image formation, but also secondary members such as a support; (b) image formation is carried out with locally selective application of thermal energy to the thermal image transfer recording medium, which is hereinafter simply referred to as the ink sheet, and remaining unused portions of the ink sheet cannot be used again; and (c) there is a case where a yellow ink sheet, a magenta ink sheet, a cyan ink sheet and a black ink sheet are individually employed in order to obtain a full-color image.
To eliminate these shortcomings, the so-called n-times-speed mode method has been proposed, by which the running speed of an ink sheet is made slower than that of an image receiving sheet, so that the ink sheet can be used repeatedly.
However, when this n-times-speed mode method is employed in the sublimation-type thermal transfer recording method, it is practically difficult to perform a multiple printing, even when the dye-content in an ink layer of the ink sheet is increased large enough for multiple-printing, because there is a problem in the thermal diffusion of sublimable inks employed.
The inventors of the present invention proposed a sublimation-type thermal image transfer recording medium with a two-layered structure (Japanese Laid-Open Patent Application 2-586) to eliminate the above-mentioned problem. More specifically, this sublimation-type thermal transfer recording medium has such a structure that a dye-transfer-contribution layer with a relatively small dye-releasing capability is provided on a dye-supply layer with a large dye-releasing capability.
By use of the sublimation-type thermal transfer recording medium with the above-mentioned structure, the dye can be speedily replenished to the dye-transfer-contribution layer from the dye-supply layer, so that the ink is transferred from the dye-transfer-contribution layer to the image receiving sheet to form images thereon.
The above-mentioned thermal image transfer recording medium proposed by the inventors of the present invention has an excellent multi-printing performance so that image formation can be performed multiple times without any substantial decrease in image density even after multiple image transfer operations are repeated. However, it is necessary to minimize the amount of the dye contained in the recording medium, which constitutes the largest percentage of the manufacturing cost of the thermal image transfer recording medium, and to improve the manufacturing method of the recording medium.
For instance, in order to form a desired dye-supply layer comprising a sufficient amount of a dye for multiple image formation by using a conventional coating method, coating must be made two or more times for the formation of the dye-supply layer. As a result, the manufacturing cost is increased.
Moreover, when an ink-ribbon-shaped sublimation-type thermal image transfer recording medium, which is in general use, is produced, if a dye-transfer layer is prepared by a microgravure printing method, which is considered to be suitable for film formation, coating irregularity is often caused at the initiation of the coating or immediately before the termination of the coating. If this takes place, such coating irregularity has to be removed from the dye-transfer layer. This inevitably increases the manufacturing cost of the recording medium.